Semiconductor modules with power semiconductor chips are used in the inverters (power converters) of the variable speed drives for the motors of electric vehicles. The elements such as insulated gate bipolar transistors (referred to as “IGBT,” hereinafter) are used as the power semiconductor chips. Because the power semiconductor chips generate heat due to high current flowing therein, a semiconductor module is used in combination with a cooler. In electric vehicles and other products with limited weight and space to attach a cooler, a liquid-cooled cooler using a circulatory coolant is used in order to improve the heat dissipation performance of the semiconductor module.
A semiconductor module has a heat dissipation base that is connected thermally to power semiconductor chips via a substrate, and the heat dissipation base is provided with cooling fins. The cooling fins are accommodated in a flow path formed between the heat dissipation base and a cooler. By flowing a pressurized cooling medium (also referred to as “coolant,” hereinafter) through the cooling fins, heat generated by the semiconductor chips is efficiently released to the cooling medium. The cooling medium that is warmed up by the heat released from the semiconductor chips is cooled by an external heat exchanger, which is then pressurized by a pump and then returned to the flow path in which the cooling fins are disposed.
This type of conventional technique is disclosed in, for example, Patent Documents 1 to 5.
Patent Document 1 discloses a cooling system that has a cooling unit accommodating fins therein, a partial structure that has a cross-sectional flow path that gradually narrows down in the short-side direction of the cooling unit and gradually expands in the long-side direction of the same, and a partial structure that has a cross-sectional flow path that gradually expands from the short side of the cooling unit and gradually narrows down from the long side of the same. Patent Document 2 discloses a cooler that has a parallel flow path configured by a large number of fine flow paths, a first header for distributing a coolant to each flow path of the parallel flow path, and a second header for merging the coolant flowing out of the parallel flow path. Patent Document 3 discloses a cooling device that has a protrusion disposed on an upper surface of a bottom part that configures a bottom surface of a coolant path, wherein the protrusion is formed by an upstream-side upward inclined surface and a downstream-side downward inclined surface. Patent Document 4 discloses a power module cooling unit in which a bottom surface of an opening of a heat sink is provided with a spacer for smoothly reducing the cross-sectional area of a flow path. Patent Document 5 discloses a semiconductor cooling device in which a flow path cover is formed in such a manner that the clearance between the flow path cover and the edges of the pin-shaped fins is wide on the inlet side and narrow on the outlet side. Patent Document 6 discloses a cooling device in which heat transfer fins are formed in such a manner that a coolant is placed higher on the inlet side than on the outlet side.
Patent Document 1: Japanese Patent Application Publication No. 2004-6811 (FIGS. 1 and 7, paragraphs 0023 to 0031, and 0056 to 0061)
Patent Document 2: Japanese Patent Application Publication No. 2001-35981 (FIGS. 1 and 2, paragraphs 0020 to 28)
Patent Document 3: Japanese Patent Application Publication No. 2008-263137 (FIG. 11, paragraphs 0035 to 0038)
Patent Document 4: Japanese Patent Application Publication No. 2001-308246 (FIG. 9, paragraph 0034)
Patent Document 5: Japanese Patent Application Publication No. 2010-153785 (FIGS. 7 and 8, paragraphs 0035 to 0037)
Patent Document 6: Japanese Patent Application Publication No. 2007-81375 (FIGS. 2, 3, 6, and 7, paragraphs 0038 to 0050)
Incidentally, in the semiconductor devices described above, the cooling fins tend to be designed to have detailed and complicated shapes in order to efficiently release the heat generated by the power semiconductor chips. The use of the cooling fins of such shapes is likely to increase pressure loss in the coolers, creating a need for a high-output pump for circulating the coolant. On the other hand, reducing the sizes of the semiconductor devices or configuring the semiconductor devices to provide high outputs, leads to an increase in the amount of heat generated per unit area, creating a need for detailed, dense cooling fins, which results in an increase of pressure loss and creating a need for a large pump.